Richard Greenland
Last week I discussed damp in floors. This week it’s about rising damp in walls.
Many older properties have no damp-proof course (DPC) in the walls, allowing rising damp to damage plaster and cause florescence (salt crystals forming on the surface) and mildew on wall surfaces. The two main ways of treating it are:
1. Injecting a chemical DPC.
2. Painting a waterproof barrier onto the wall beneath the plaster.
Number 1 is best for large areas as it avoids lots of expensive re-plastering. It’s a task for a specialist company, they inject a silicone-based chemical into the wall base at very high pressure. They should provide a guarantee, and it’s very normal to have to call them back to re-inject small areas where the DPC hasn’t fully taken.
Number 2 is suitable for isolated areas. The plaster is chipped back to bare masonry, and two coats minimum of waterproofing agent are applied. The most popular waterproofers are bitumen emulsion and Vandex.
bitumen paint being applied
Bitumen is old-fashioned, very messy, but highly effective. It won’t set on very wet walls. (You need tanking for this – beyond the scope of this blog but I may write another). Care must be taken not to leave any tiny holes with the first coat. When this is dry, a second coat is applied, then while still wet, ‘blinded’ with sharp sand thrown against it. This provides a key for the render.
Vandex is a cementatious slurry. It’s micro-porous and less messy, but in my experience more easily penetrated by efflorescence than bitumen.
The wall is then re-plastered with sand and cement render with a waterproofing additive. The additive gives belt-and-braces protection. Gypsum plaster is very hydrophilic (attractive of water) and only suitable for the top finish coat.
Any screw-holes in treated walls should be injected with silicone sealant (bathroom sealant will do) before the wall-plug and screw are put in, to seal the hole from damp.
If damp patches appear over 1.2 metres above ground level it’s not usually rising damp, but penetrating damp or condensation. Landlords often blame tenants for condensation problems, but there are loads of ways to make your properties much less prone to it. That’ll be the subject of next week’s blog.
Not the best advice,
1.rising damp does not cause mildew, thats usually caused by condensation.
2. a DPC injection on its own is no good as it doesnt address the salt deposits within the plaster.
3. a waterproof coating on the brickwork is a waste of money and can actually lead to problems if a non breathable substance is used.
4. tanking a wall that is effected by rising damp will leave a dry decorative finish but is not very cost effective.
5. the answer to true rising damp is simple, chemical DPC injection to the base of the wall then a 2 coat sand and cement render incorporating a waterproof chemical(sika1) applied directly to the brickwork. this is the most cost effective and done correctly will last many many years if not the life of the building.
Nick,
1. Any damp may cause mildew, regardless of the source.
2. Carbonate and silicate salts are just as likely to be in the walls as in the plaster. Once the wall dries there is no further possibility of evaporation deposits.
3. How?
4. That’s why I said it was beyond the scope of this blog and only for very wet walls. I might add for walls subject to severe hydrostatic pressure due to location underground. The other practicable solution there is to dig a trench + retaining wall. Both are expensive.
5. This is a combination of my methods 1 & 2, but omitting the bitumen/ Vandex. I have used them in combination many times when we don’t have months (or years!) for the walls to dry out. Some I did 30 years ago are still good
Having gone to all that trouble it seems a bit remiss to leave out the Vandex?